1. Field of the Invention
This invention relates to a brake fluid pressure control apparatus in a skid control system for a vehicle having at least one wheel and a brake for the wheel in which the brake fluid pressure to the wheel cylinder of the brake for the wheel is controlled in accordance with the rotational condition or skid condition of the wheel, and more particularly to a brake fluid pressure control apparatus of the type in which, when the brake for the wheel is relieved, brake fluid discharged through a fluid pressure control valve device from the wheel cylinder of the brake into a hydraulic reservoir is returned to a pressure fluid supply conduit connected to a master cylinder by a fluid pump with motor, and which is provided with a pump fail detecting circuit for detecting the fail of the fluid pump with motor.
2. Description of the Prior Art
Recently, various kinds of skid control systems have been developed for a vehicle having at least one wheel and a brake for the wheel, by which skid control operation for the wheel is performed to avoid a locked wheel condition and obtain good braking performance on any road. In any of the skid control systems, the rotational condition or skid condition of the wheel such as deceleration, slip and acceleration is measured by the control unit which receives the detecting signal of the wheel speed sensor which is associated with the wheel for detecting the rotational speed of the wheel. The brake fluid pressure to the brake for the wheel is controlled on the basis of the measurement of the control unit.
Methods for measuring the rotational condition or skid condition of the wheel can be roughly classified into three. In the first method, slip of the wheel is obtained from the vehicle speed and the wheelspeed, and the obtained slip of the wheel is compared with a predetermined slip. In the second method, acceleration or deceleration of the wheel is obtained, and the obtained acceleration or deceleration of the wheel is compared with a predetermined acceleration or deceleration. And in the third method, slip and acceleration or deceleration of the wheel are obtained from the vehicle speed and wheel speed, and the obtained slip and acceleration or deceleration of the wheel are compared with the predetermined slip and acceleration or deceleration.
A brake fluid pressure control apparatus is arranged between a master cylinder as a brake pressure generating member and the brake for the wheel. Control signals from the control unit as the measurement result are supplied to the brake fluid pressure control apparatus for increasing and decreasing, or increasing, maintaining at constant, and decreasing the brake pressure to the brake for the wheel.
Such a brake fluid pressure control apparatus is known that includes a brake fluid pressure control valve device to which the control signals from the control unit are supplied, to control the brake fluid pressure to the wheel cylinder of the brake for the wheel, a hydraulic reservoir for reserving the brake fluid discharged through the brake fluid pressure control valve device from the wheel cylinder of the brake, when relieved, and a fluid pump with motor receiving a drive signal from the control unit for returning the brake fluid from the hydraulic reservoir into the pressure fluid supply conduit connecting the master cylinder and the brake fluid pressure control valve device.
In the above brake fluid pressure control apparatus, the brake fluid from the wheel cylinder of the brake is discharged into the hydraulic reservoir to relieve the brake for the wheel, and at once, it is returned to the pressure fluid supply conduit by the fluid pump with motor. When the brake for the wheel is reapplied, the brake fluid is supplied through the brake fluid pressure control valve device into the wheel cylinder of the brake from the pressure fluid supply conduit.
However, if the fluid pump with motor fails or is wrong, it does not operates with the drive signal supplied from the control unit. The brake fluid cannot be returned to the pressure fluid supply conduit from the hydraulic reservoir. Accordingly, whenever the brake for the wheel is reapplied, brake fluid should be supplied from the master cylinder into the wheel cylinder. The displacement of the brake pedal increases. At last, the displacement of the piston of the master cylinder might reach the full stroke, and it might become impossible to reapply the brake.
On the other hand, when the brake fluid cannot be returned to the pressure fluid supply conduit from the hydraulic reservoir, the volume of the brake fluid in the hydraulic reservoir increases whenever the brake for the wheel is relieved. At last, it might reach the capacity of the hydraulic reservoir and it might become impossible to relieve the brake.
Accordingly, if the fluid pump with motor fails, there is the fear that the skid control system becomes inoperative.